1. Field of the Invention
The present invention relates to the technology of controlling a power amplifier, and more specifically to a power amplifier used in a wireless transmission/reception device in a time division duplex system.
2. Description of the Related Art
In the time division duplex system (hereinafter referred to as TDD for short), a communication path is divided by a time axis, a transmission/reception switch is quickly made, and a frequency band is shared between a transmitter and a receiver. As shown in FIGS. 1A and 1B, a switch and a circulator are provided immediately below an antenna so that communications can be performed by switching between transmission and reception.
In the TDD system, a frequency band is shared between transmission and reception. Therefore, it is necessary that isolation is completely secured between a transmission unit and a reception unit. When the isolation is not sufficient, there occur problems such as a leak of a transmission signal to the reception unit, saturating a low noise amplifier, etc. For example, when a transmission/reception changeover switch is provided, the transmission/reception switch time is shorter at a higher transmission rate. Then, with a shorter transmission/reception switch time, it becomes more difficult to secure the isolation.
To secure the isolation, it is desired that the power amplifier of the transmission unit is turned on and the low noise amplifier of the reception unit is turned off during transmission, and that the power amplifier of the transmission unit is turned off and the low noise amplifier of the reception unit is turned on during reception. It is necessary from the viewpoint of improving power efficiency.
Assume a power amplifier using a field effect transistor. In this case, since a power current passing through a drain is large in the power amplifier, the current value is not immediately set to zero due to transient response although the voltage to be applied to the drain is switched off. Therefore, the residual signal is leaked to the reception side, thereby possible degrading the reception performance.
As the technology for controlling the on/off state of the power amplifier to solve the above-mentioned problems, for example, Japanese Published Patent Application No. H9-74321 discloses the technology of providing an emitter unit of a high frequency amplification transistor with a switching element together with an emitter resistor and setting an active state during reception and passive state during transmission, thereby turning on/off a high frequency amplification transistor.
As another technique for controlling the on/off state of the power amplifier, for example, Japanese Published Patent Application No. H4-373317 discloses the technology of providing a transmitting power amplifier and a receiving power amplifier to turn off the transmitting power amplifier by the control voltage during reception and turn on the receiving power amplifier. Then, the receiving power amplifier is turned off and the transmitting power amplifier is turned on by the control voltage during transmission.
The methods disclosed by the above-mentioned Japanese Published Patent Application No. H9-74321 and Japanese Published Patent Application No. H4-373317 switch on and off the transmitting power amplifier to suppress a leak of the transmission signal to the reception unit in the TDD system. However, as described above, since the current passing through the drain of the power amplifier is large, it is not easy to quickly switch on/off the power amplifier by the switch circuit.